Please refer to FIG. 1, a conventional printed circuit board (PCB) which can be served as an integrated circuit (IC) chip carrier, typically has a substrate 90. The substrate 90 has a top surface and a bottom surface wherein either surface, or both surfaces, has a conductive layout pattern and a solder mask 92 which is coated on the surface of the substrate 90 and covered over the conductive pattern. The conductive pattern usually has bonding pads 94 for electrically coupling the PCB to such devices as electronic elements, e.g. chips, and electronic carriers, e.g. mother circuit board, by means of solder balls or bonding wires. The solder mask 92 provides at predetermined positions thereof with a plurality of openings 922 corresponding in locations to the bonding pads 94 such that the bonding pads 94 can be exposed outside. The openings 922 are formed by etching the solder mask 92, and they usually have larger dimensions at top ends thereof and smaller dimensions at bottom ends thereof. As shown in FIG. 1, each of the openings 922 can receive a solder ball 96 therein. The solder ball 96 is soldered on the bonding pad 94 of the conductive pattern by means of reflowing treatment. The solder ball 96 has a bump portion 962 extruded out of the opening 922 for coupling the PCB to a predetermined device. The solder ball 96 has the other portions which are connected to the solder mask 92 and the bonding pad 94 at the regions labeled 97, 98 and 99.
It is obvious to understand that the solder ball 96 has a weak capacity to be secured in the opening 922, because of the solder ball 96 is mainly contacted with the solder mask 92 at the regions labeled 97 and 98. However, the solder mask 92 is made of resin material such as epoxy resin, which has a weak interface-connected strength to bond the metallic solder ball 96. So, the solder balls 96 are easy to loose from the openings 922 while an unexpectable force exerted onto the solder balls 96.